Method of producing ceramic

ABSTRACT

A ceramic powder of grains having a shape-anisotropy is mixed with a calcined or uncalcined (or both) powder of a ceramic raw material. A ceramic slurry containing the produced mixed powder, a solvent and a binder is prepared and the ceramic slurry is formed into a sheet. A laminate comprising a plurality of the sheets laminated to each other is uniaxially pressed to form an oriented product in such a manner that the length of the laminate in the direction parallel to the pressing axis becomes larger than that before the pressing, and the area of a plane perpendicular to the pressing axis of the laminate becomes larger than that before the pressing. The oriented formed product is fired and sintered.

[0001] This is a continuation of application Ser. No. 10,141,363 filedMay 7, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of producing a ceramicand, more particularly, to a method of producing an oriented ceramicwhich can be especially used as an electronic material such as apiezoelectric material or the like.

[0004] 2. Description of the Related Art

[0005] According to one method of producing ceramics in the field of thepresent invention, ceramic green sheets are laminated, press bonded toeach other and fired. The ceramic green sheets are pressed in thismethod in such a manner that the area of the respective ceramic greensheets in the direction perpendicular to the pressing axis is preventedfrom increasing. The crystal grains of the ceramic obtained by thismethod are not oriented.

[0006] On the other hand, it is known that oriented ceramics in whichthe crystal grains are oriented are especially useful as electronicmaterials such as piezoelectric materials or the like. For example, asdescribed in the report by T. Takenaka, et al., the orientation of alayered perovskite compound ceramic such as Na_(0.5)Bi_(4.5)Ti₄O₁₅ orthe like as a piezoelectric material caused the electromechanicalcoupling coefficient for the thickness longitudinal fundamentalvibration of a columnar vibrator to increase to about 2.2 times of thatof the ordinary not-oriented ceramic (Sensor and Materials. Vol. 1, 35(1988)). S. Jin et al reported that as a superconductor material, anoriented YBa₂Cu₃O₇-δ ceramic was prepared, and the critical currentdensity thereof was increased to about 12 times of that of thenon-oriented ceramic (Physical Review B, vol. 37, No. 13, 7850 (1988)).It will be appreciated that orientation means the state of crystalgrains having a large shape-anisotropy in which the directions of thegrains are the same as a whole.

[0007] As methods of producing oriented ceramics, hot forging, TemplatedGrain Growth (TGG), and so forth have been employed.

[0008] Oriented ceramics having high orientation degrees can be obtainedby the hot forging method. T. Takenaka et al produced an orientedceramic of Na_(0.5)Bi_(4.5)Ti₄O₁₅ by the hot forging method. Accordingto this method, a formed product is heat-treated (fired) while it ispressed. The orientation degree of the produced oriented ceramic,measured by the Lotgering method, was 98%. However, the hot forgingmethod needs to employ a special heat-treatment apparatus suitable forpress-firing and, moreover, is a batch-process heat-treatment. Thus,this method is expensive and unsuitable for mass production.

[0009] Seong-Hyon Hong et al produced an oriented ceramic ofBi₄(Ti_(3.06)Nb_(0.04))O₁₂ by the TGG method. Here, the ceramic crystalgrains having a shape-anisotropy are mixed prior to forming. Theorientation degree of the oriented ceramic obtained by this method,measured by the Lotgering method, was 96%, and the piezoelectricconstant d33 thereof was enhanced to about 1.5 times of that of thenon-oriented ceramic (J. Am. Ceram. Soc., vol. 83, 113 (2000)). It isunnecessary to press-fire by a batch-process according to the TGGmethod, and therefore, this method is suitable for mass production.However, the orientation degree of the crystal grains of a ceramicproduced by the TGG method is low compared to that by the hot forgingmethod.

[0010] To enhance the characteristics of a ceramic such as anelectromechanical coupling coefficient by orienting the ceramic, it isnecessary to realize a still higher orientation degree. In general, itis more difficult to produce highly-oriented ceramics by the TGG methodcompared to the production by the hot forging method.

[0011] The inventors compared a non-oriented ceramic prepared bypress-bonding a laminate and then firing with the hot forging and TGGmethods using CaBi₄Ti₄O₁₅+0.5% by weight MnCO₃. Table 1 shows thecomparison results. TABLE 1 Press-bonded Hot laminate is fired forgingTGG* Orientation degree 0%   98%   91% Electromechanical 15% 35.1% 30.5%coupling coefficient at thickness shearing vibration

[0012] As seen in Table 1, the crystal grains of the ceramics producedby the hot forging and TGG methods are oriented in contrast to theceramic produced by the prior art method of firing a press-bondedlaminate. The orientation degree of the ceramic or produced by the TGGmethod is lower, and also, the enhancement of the characteristic issmaller, compared to those by the hot forging method.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is a main object of the present invention toprovide a method of producing a ceramic by which an ordinary bakingfurnace can be used for firing, and in the case of the same materialsbeing used, an oriented ceramic having an orientation degree higher thanthat made by the TGG method can be produced.

[0014] It is another object of the present invention to provide a methodof producing a ceramic by which an ordinary baking furnace can be usedfor firing, and in the case of the same materials being used, anoriented ceramic having an orientation degree higher than that by theTGG method and substantially equal to that by the hot forging method canbe produced.

[0015] It is still another object of the present invention to provide amethod of producing a ceramic by which an ordinary baking furnace can beused for firing, and in the case of the same materials being used, anoriented ceramic which has a higher orientation degree and a highersintering density than those by the TGG method can be produced.

[0016] Specifically, the present invention provides a method ofproducing a ceramic comprising the steps: preparing ceramic slurrycontaining a powder of ceramic crystal grains having a shape-isotropymixed with a powder of a ceramic raw material or a calcined powder of aceramic raw material, or both; forming the ceramic slurry to produce aformed product; uniaxially pressing the formed product so that thelength of the formed product in the direction parallel to the pressingaxis is decreased compared to that before the pressing, and the area ofa plane perpendicular to the pressing axis of the formed product isincreased compared to that before the pressing, whereby an orientedformed product is produced; and firing the oriented formed product tosinter it.

[0017] Preferably, the length of the oriented formed product in thedirection parallel to the pressing axis is up to about half of thelength of the formed product before pressing.

[0018] Also, preferably, the amount of the ceramic crystal grains havinga shape-anisotropy is in the range of about 25 to 52% by weight based on100% by weight of the mixed powder.

[0019] Furthermore, preferably, the ceramic crystal grains having ashape-anisotropy are flat, and the aspect ratio is in the range of about5 to 10. The aspect ratio is the ratio of the maximum size of a ceramiccrystal grain to the height thereof.

[0020] Preferably, the ceramic crystal grains having a shape-anisotropyhave a layered perovskite crystal structure.

[0021] According to the method of producing a ceramic of the presentinvention, an ordinary baking furnace can be used for firing. When thesame materials are used, an oriented ceramic having a higher orientationdegree than that by the TGG method can be obtained. Therefore, theproduction cost of the ceramic can be reduced, and also, a ceramichaving a higher orientation degree compared to that by the TGG methodcan be produced.

[0022] When the length of the oriented formed product in the directionparallel to the pressing axis is up to about half of the length of theformed product before pressing, a ceramic having a still higherorientation degree, e.g., an orientation degree substantially equal tothat by the hot forging method, can be produced.

[0023] Moreover, when the amount of the ceramic crystal grains having ashape-anisotropy is in the range of about 25 to 52% by weight based on100% by weight of the mixed powder, a ceramic having a high orientationdegree and a high sintering density can be obtained.

[0024] Also, when the ceramic crystal grains having a shape-anisotropyare flat, and the aspect ratio (ratio of the maximum size thereof to theheight) is in the range of about 5 to 10, a ceramic having a highorientation degree can be obtained. If the aspect ratio is higher thanabout 10, the density of the ceramic becomes low.

[0025] Moreover, when the ceramic crystal grains having ashape-anisotropy have a layered perovskite crystal structure, anoriented ceramic having a remarkably higher orientation degree and asuperior piezoelectric characteristic can be produced. Examples of thematerial having the layered perovskite crystal structure includes BiWO₆,CaBi₂Nb₂O₉, SrBi₂Nb₂O₉, BaBi₂Nb₂O₉, PbBi₂Nb₂O₉, CaBi₂Ta₂O₉, SrBi₂Ta₂O₉,BaBi₂Ta₂O₉, PbBi₂Ta₂O₉, Bi₃TiNbO₉, Bi₃TiTaO₉, Bi₄Ti₃O₁₂, SrBi₃Ti₂NbO₁₂,BaBi₂Ti₂NbO₁₂, PbBi₃Ti₂NbO₁₂, CaBi₄Ti₄O₁₅, SrBi₄Ti₄O₁₅, BaBi₄Ti₄O₁₅,PbBi₄Ti₄O₁₅, Na_(0.5)Bi_(4.5)Ti₄O₁₅, K_(0.5)Bi_(4.5)Ti₄O₁₆,Ca₂Bi₄Ti₅O₁₈, Sr₂Bi₄Ti₅O₁₈, Ba₂Bi₄Ti₅O₁₈, Pb₂Bi₄Ti₅O₁₈, BiTiWO₁₈,Bi₇Ti₄NbO₂₁, Bi₁₀Ti₂W₃O₃₀, and combinations of at least two of thesematerials.

[0026] The above-mentioned objects of the present invention, and alsothe characteristics and the advantages thereof will be clarified in thedetailed description of the preferred modes of carrying out theinvention and the examples of the invention, which is made hereinunderwith reference to the drawings attached thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 illustrates a step contained in an example of the methodfor producing a ceramic according to the present invention;

[0028]FIG. 2 illustrates a step contained in an example of the methodfor producing a ceramic of the prior art;

[0029]FIG. 3 is a graph showing relations between the contents of thesheet-shaped ceramic powder and the orientation degrees of the samplesof the example and prepared by the TGG method;

[0030]FIG. 4 is a graph showing relations between the contents of thesheet-shaped ceramic powder and the densities of the samples of theexample and prepared by the TGG method; and

[0031]FIG. 5 is a graph showing relations between the aspect ratios andthe orientation degrees of the samples of the example and prepared bythe TGG method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE

[0032] As starting materials, Bi₂O₃, TiO₂, CaCO₃ and MnCO₃ wereprepared. These materials were weighed out so that a composition ofCaBi₄Ti₄O₁₅+0.5% by weight of MnCO₃, can be produced, and wet-mixed forabout 16 hours by means of a ball mill. The obtained mixture was driedand calcined at 900° C. for 2 hours to obtain calcined powder of theceramic materials.

[0033] A portion of the calcined powder and NaCl were mixed at a mixingweight ratio of 1:1, and heat-treated (fired) at a temperature of 950 to1050° C. for 10 hours. The NaCl was removed from the fired product toobtain a ceramic powder of CaBi₄Ti₄O₁₅. Scanning electron microscopy ofthe powder showed that it was anisotropic in shape and had a sheet-likeshape. Moreover, the aspect ratio of the sheet-shaped ceramic powder,that is, the ratio of the height to the maximum size was about 10.

[0034] A mixed powder comprising 50 parts by weight of the sheet-shapedceramic powder and 50 parts by weight of the above-described calcinedpowder, an organic binder, a dispersant, an antifoaming agent and asurfactant were mixed to obtain ceramic slurry. The ceramic slurry wasformed by a doctor blade method into sheets as formed products. Thethickness of the sheets were in the range of 40 to 100 μm. The sheetswere overlaid so that the total thicknesses of the resulting laminateswere 1.25 mm, 1.7 mm, 2 mm and 3.3 mm. The sheets were then uniaxiallypressed to be securely fixed to each other. Thus, oriented formedproducts as samples were obtained.

[0035] Especially referring to the uniaxially pressing, each laminate Lcomprising the overlaid sheets was placed into a metallic mold 10 andthen pressed, as shown in FIG. 1. In this case, the size of the metallicmould 10 was adjusted so that the press thickness-reduction ratios ofthe laminates L having thicknesses of 1.25 mm, 1.7 mm, 2 mm and 3.3 mmwere 0.8, 0.6, 0.5 and 0.3, respectively, and the thicknesses after thepress-bonding of the laminates L were about 1 mm. The press thicknessreduction ratio is defined by the following formula

press thickness reduction ratio=H ₁ /H ₀

[0036] in which H₀ represents the thickness of the laminate comprisingoverlaid sheets before pressing, and H₁ is the thickness of the laminateafter the pressing.

[0037] For comparison, a sample was prepared by a TGG method. That is, ametallic mold 1 having a size equal to the sheets was used as shown inFIG. 2. The laminate L comprising the sheets overlaid on each other asdescribed above was placed in the mold, and pressed so that the lengthof the laminate L in the direction parallel to the pressing axis and thearea of the plane of the laminate L perpendicular to the pressingdirection after the pressing were not changed from those before thepressing. In other words, the sheets were securely bonded to each otherso as to have a press thickness-reduction ratio of 1.0.

[0038] Thereafter, the respective samples were heat-treated (fired) at1150° C. for 2 hours to be sintered. The orientation degree at thesurface of each sample was measured by the Lotgering method. TheLotgering method is one of the techniques for measuring the orientationof a sample. That is, a ratio P₀ is determined as follows:

ration P ₀ ={ΣI(001)/ΣI(hkl)}

[0039] in which ΣI (hkl) represents the sum of the reflectionintensities I (hkl) at the respective crystal planes (hkl) of anon-oriented sample, and ΣI(001) represents the sum of the reflectionintensities I (001) at the (001) planes. A ratio P for an orientedsample is determined in a similar manner:

P={ΣI(001)/ΣI(hkl)}

[0040] The orientation degree F is then determined using P₀ and P asfollows.

F(%)={(P−P ₀)/(1−P ₀)}×100

[0041] For comparison, samples having the same composition as those ofthe above-described Example were prepared by a hot forging method. Inthe hot forging method, a uniaxial pressing force is applied to a samplewhile it is being fired. In this case, the same calcined powder as thatof the above-described Example was mixed with an organic binder. Thesample was press-formed into a columnar shape with a diameter of 17 mmand a height of 8 mm. The sample was heat-treated (fired) at 1150° C.for 2 hours. In this heat-treatment, the sample was uniaxially pressedat a total pressure of about 500 kg.

[0042] Table 2 shows the relationship between the pressing thicknessreduction ratios of the samples prepared as described above and theorientation degrees thereof. The orientation degree of the sampleprepared by the hot forging method was 98%. TABLE 2 pressing thickness-Orientation degree reduction ratio (%) 1.0* 91 0.8 95 0.6 96 0.5 98 0.398

[0043] As shown in Table 2, the orientation degrees of all the samplesaccording to the present invention were at least 95%.

[0044] The sample prepared according to the TGG method pressed in such amanner that the length thereof in the direction parallel to the pressingaxis and the area of the surface thereof perpendicular to the pressingdirection after pressing were not changed from those before thepressing, that is, at a pressing thickness reduction of 1.0, had anorientation degree of 91%.

[0045] As seen in Table 2, the oriented ceramic samples according to thepresent invention and prepared at a pressing thickness reduction ratioof up to about 0.5, that is, prepared in such a manner that the lengthof an orientated formed-product in the direction parallel to thepressing axis was up to about half of the product to be formed, hadorientation degrees that were almost equal to those of the orientatedceramic samples obtained by the hot forging method.

[0046] For reference, Table 3 shows the relation between the orientationdegrees of the oriented ceramic of CaBi₄Ti₄O₁₅ and the electromechanicalcoefficients at thickness shear mode vibration. TABLE 3electromechanical coupling orientation degree coefficient (%) atthickness (%) shear mode vibration 91 30.5 93 31.1 95 31.9 96 32.7 9833.8

[0047] As apparently seen in Table 3, the electromechanical couplingcoefficient caused at thickness shear vibration was larger as theorientation degree of the oriented ceramic of CaBi₄Ti₄O₁₅ was higher.

[0048] In other samples of the above-described Example, the content ofthe sheet-shaped ceramic powder were set at 10% by weight, 25% byweight, 45% by weight, 50% by weight, 52% by weight and 60% by weight.The orientation degrees and the densities of these samples weremeasured. In this case, the aspect ratio of the sheet-shaped ceramicpowder was 10, and the pressing thickness reduction ratio of the sampleswas 0.5. Table 4 shows the results. TABLE 4 content of sheet-shapedceramic powder (% by orientation degree Density weight) (%) (g/cm³) 1060 6.9 25 85 7.1 45 95 7.2 50 98 7.2 52 98 7.15 60 98 6.7

[0049] Similarly, samples were prepared by the TGG method in the samemanner as that of the samples of Table 4 except that the pressingthickness reduction ratio was 1.0, and the orientation degrees and thedensities were measured. Table 5 shows the results. TABLE 5 content ofsheet- shaped ceramic powder Orientation degree density (% by weight)(%) (g/cm³) 10 47 6.8 25 74 6.8 45 85 7.0 50 91 7.0 52 91 6.95 60 91 6.5

[0050]FIG. 3 is a graph which illustrates the results of Tables 4 and 5with respect to the relations between the content of the sheet-shapedceramic powder and the orientation degrees of the samples.

[0051] As apparent from Tables 4 and 5 and the graph of FIG. 3, theorientation degrees of the samples of the Example were higher than thoseof the samples obtained by the TGC method. Table 4 and the graph of FIG.3 show that when the content of the sheet-shaped ceramic powder was 25%by weight or higher in Example, the orientation degree was 85% orhigher. Moreover, orientation degree was 95% or higher and when thecontent of the sheet-shaped ceramic powder was 50% by weight or higher,the orientation degree was 98% or higher.

[0052]FIG. 4 is a graph which illustrates the results of Tables 4 and 5with respect to the relations between the content of the sheet-shapedceramic powder and the densities of the samples.

[0053] As seen in Tables 4 and 5 and the graph of FIG. 4, the densitiesof the samples in Example were higher than those of the samples preparedby the TGG method. Table 4 and the graph of FIG. 4 show that when thecontent of the sheet-shaped ceramic powder in the samples of the examplewere in the range of about 25 to 52% by weight, the densities of thesamples were large, that is, 7.1 g/cm³ or higher. When the content ofthe sheet-shaped ceramic powder were in the range of about 45 to 50% byweight, the densities of the samples were still larger. Therefore,preferably, the content of the sheet-shaped ceramic powder is in therange of about 25 to 52% by weight based on 100% by weight of the powderof the sheet-shaped ceramic powder mixed with the calcined powder of theceramic raw material, and more preferably, in the range of about 45 to50% by weight.

[0054] The orientation degrees of the same samples as those of Exampleexcept that the aspect ratios of the sheet-shaped ceramic powder were 2,4, 5 and 10 were measured. In this case, the content of the sheet-shapedceramic powder was 50% by weight based on 100% by weight of the power ofthe sheet-shaped ceramic powder mixed with the calcined powder of theceramic raw material, and the pressing thickness reduction ratios of thesamples were 0.5. Table 6 shows the results. TABLE 6 orientation degreeaspect ratio (%) 2 70 4 91 5 98 10 98

[0055] Using the TGC method, samples which were the same as those justshown except that the pressing thickness reduction ratios were 1.0 wereprepared. The orientation degrees of the samples were measured. Table 7shows the results. TABLE 7 orientation degree aspect ratio (%) 2 62 4 835 91 10 91

[0056]FIG. 5 is a graph which illustrates the results of Tables 6 and 7with respect to the relations between the aspect ratios of thesheet-shaped ceramic powder and the orientation degrees of the samples.

[0057] As shown in Tables 6 and 7 and the graph of FIG. 5, theorientation degrees of the samples of Example were higher than those ofthe samples prepared by the TGG method. Furthermore, as seen in Table 6and the graph of FIG. 5, when the aspect ratio of the sheet-shapedceramic powder was about 4 or higher, the orientation degree was 91% orhigher. When the aspect ratio was about 10 or higher, the density of theceramic was decreased. Accordingly, preferably, the aspect ratio of thesheet-shaped ceramic powder is in the range of about 4 to 10, and morepreferably, in the range of about 5 to 10.

[0058] In the above-described Example, a sheet-shaped ceramic powder isused in the mixed powder. According to the present invention, however,ceramic crystal grains having another shape-anisotropy may be usedinstead of the sheet-shaped ceramic powder.

[0059] Moreover, a calcined powder of the ceramic raw material is usedin the mixed powder in the above-described Example. According to thepresent invention, a powder of the ceramic raw material may be usedinstead of the calcined powder of the ceramic raw material. Furthermore,the powder of the ceramic raw material may be used together with thecalcined powder of the ceramic raw material.

[0060] In the above-described Example, a piezoelectric material is used.However, the type of ceramic according to the present invention is notrestricted to such a material.

[0061] Moreover, the product is formed by the doctor blade method in theabove-described Example. According to the present invention, the productmay be formed by extrusion, anisotropically pressing, rolling, or thelike.

[0062] According to the present invention, an ordinary baking furnacecan be used for the firing. When the materials being fired are the same,an oriented ceramic having a higher orientation degree than thatproduced by the TGG method can be produced and the orientation degreecan be substantially equal to that of an oriented ceramic produced bythe hot forging method can be produced.

[0063] Furthermore, according to the present invention, an orientedceramic having a higher orientation degree and a higher sinteringdensity than that produced by the TGG method can be produced.

What is claimed:
 1. A method of producing a ceramic comprising:preparing a ceramic slurry comprising a powder of ceramic crystal grainshaving a shape-anisotropy mixed with a ceramic raw material powder or acalcined ceramic raw material powder, or both; forming the ceramicslurry to produce a sheet product; uniaxially pressing the sheet productso that the length of the product in the direction parallel to thepressing axis is decreased compared to that before the pressing, and thearea of a plane perpendicular to the pressing axis of the product isincreased compared to that before the pressing, whereby an orientedformed product is produced; and sintering the oriented formed product.2. A method of producing a ceramic according to claim 1, wherein thepressing is such that the length of the oriented formed product in thedirection parallel to the pressing axis is decreased up to about half ofthe length of the product before pressing.
 3. A method of producing aceramic according to claim 2, wherein the amount of the ceramic crystalgrains having a shape-anisotropy are in the range of about 25 to 52% byweight based on 100% by weight of the mixed powder.
 4. A method ofproducing a ceramic according to claim 3, wherein the ceramic crystalgrains having a shape-anisotropy are flat, and the aspect ratio is inthe range of about 4 to
 10. 5. A method of producing a ceramic accordingto claim 4, wherein the aspect ratio is in the range of about 5 to 10.6. A method of producing a ceramic according to claim 5, wherein theamount of the ceramic crystal grains having a shape-anisotropy are inthe range of about 45 to 50% by weight based on 100% by weight of themixed powder.
 7. A method of producing a ceramic according to claim 1,wherein the amount of the ceramic crystal grains having ashape-anisotropy are in the range of about 25 to 52% by weight based on100% by weight of the mixed powder.
 8. A method of producing a ceramicaccording to claim 1, wherein the ceramic crystal grains having ashape-anisotropy are flat, and the aspect ratio is in the range of about4 to
 10. 9. A method of producing a ceramic according to claim 1,wherein the ceramic crystal grains having a shape-anisotropy are flat,and the aspect ratio is in the range of about 5 to
 10. 10. A method ofproducing a ceramic according to claim 1, wherein the amount of theceramic crystal grains having a shape-anisotropy are in the range ofabout 45 to 50% by weight based on 100% by weight of the mixed powder.11. A method of producing a ceramic according to claim 1, wherein thepowder mixture is of ceramic crystal grains having a shape-anisotropymixed with a ceramic raw material powder.
 12. A method of producing aceramic according to claim 1, wherein the powder mixture is of ceramiccrystal grains having a shape-anisotropy mixed with a calcined ceramicraw material powder.
 13. A method of producing a ceramic according toclaim 1, wherein the powder mixture is of ceramic crystal grains havinga shape-anisotropy mixed with a calcined ceramic raw material powder anda ceramic raw material powder.